Like John S says more info required.!! All that John asked plus gantry weight or material/design plus type of linear bearings used.
Robins reasoning is sound enough but really without the full picture then can't be answered accurately.

Personaly I know of a 8x4 plasma that use's 3Nm motors geared 2:1 without any issue's but that use's R&P which hisn't the most efficient, also use's some crappy home made linear bearings.!!